If RCU priority boosting is to be meaningful, callback invocation mustbe boosted in addition to preempted RCU readers. Otherwise, in presenceof CPU real-time threads, the grace period ends, but the callbacks don'tget invoked. If the callbacks don't get invoked, the associated memorydoesn't get freed, so the system is still subject to OOM.

But it is not reasonable to priority-boost RCU_SOFTIRQ, so this commitmoves the callback invocations to a kthread, which can be boosted easily.